Chances are good you have a vent too. It's in the wall. Please confirm.

I've looked into this before. Here is what I know.
They are all moveable: Vent, supply pipes, and sink drain pipe.
The drain perforation in the slab stays right where it is now.
For 15 inches you can leave the vent as is, and lengthen only the sink drain.
To lengthen the sink drain, there are a couple ways.
One is to lengthen the P trap's tailpiece.
Another is to route the sink drain differently - e.g. like for an ADA sink.
Search "LKAD35" or "LK35 Drain Elbow" or "Tight radius elbow" or "Short radius elbow". Or you can use a long sweep elbow.

davidro1 said, quote: "For 15 inches you can leave the vent as is, and lengthen only the sink drain.
To lengthen the sink drain, there are a couple ways.
One is to lengthen the P trap's tailpiece.
Another is to route the sink drain differently - e.g. like for an ADA sink.
Search "LKAD35" or "LK35 Drain Elbow" or "Tight radius elbow" or "Short radius elbow". Or you can use a long sweep elbow."

WRONG! Not only is that suggestion outrageously expensive, it may prove to be code prohibited.

First of all, you cannot just arbitrarily lengthen the sink drain because the codes are very specific about the maximum permissible length from the trap weir to the vent opening.

Under the IRC (International Residential Code) an 1-1/4" line may run a maximum of 5ft and a 1-1/3" line may run a maximum of 6ft from the trap weir to the vent.

Under the UPC (Uniform Plumbing Code) an 1-1/4" may run 2'6" and a 1-1/2" may run 3'6" from trap weir to vent opening.

Note: The "trap weir" is the physical point on the trap where the U section transitions to horizontal on the discharge side of the trap. (It is the exact point where water passing through the trap would spill out of the U section and into the horizontal tailpiece.)

Both the IRC and the UPC absolutely forbid adding an extension tailpiece on the trap tailpiece. The codes allow a maximum of one slip joint on the discharge side of a trap after the trap weir. That single slip joint would be the trap adapter where the tailpiece connects to the fixture arm extending from the wall

Before you move your sink you must first determine the exact location of the vent.

You should have a "Trim Escutcheon" (Chrome trim ring) were the fixture arm (drain pipe) extends out of the wall. slip the trim escutcheon slightly forward and you should find that the hole in the wall is larger than the pipe. You should be able to peek in the hole to see if the the fixture arm extending through the wall is connected to a tee in the wall. If it is connected to a tee and there is a vertical line going up from the tee, the vertical riser above the tee is the vent line. (Note: This is the most common configuration because we can usually vent the entire bathroom group from this one vent.)

If the fixture arm turns horizontally in the wall and the vent line is not there, you must then determine where the vent is to figure out if the distance from the vent to your new sink will be within the maximum allowable lengths as were mentioned above.

If the vent line is connected to a tee behind the existing fixture arm you could then cut the trap adapter off the end of the fixture arm and attach a 1/4bend (90deg elbow), 1/8th bend (45deg elbow) or a 1/6th bend (60deg elbow) then install an extension piece of pipe on the fixture arm and finally attach a trap adapter to the end of the fixture arm where it would meet the tailpiece of the new trap.

The IRC permits an unlimited number of AAV's (Air Admittance Valve -commonly called a Studor vent) therefore if you are under the IRC if the fixture arm turns horizontal and you cannot determine the length from the trap weir to the vent you could install an AAV on the extended portion of the fixture arm.

The UPC only permits a maximum of one AAV per structure, with the expressed consent of the local AHJ. (Normally they will only grant permission for an AAV to vent a sink on a kitchen island.)

Personal note- I hope I got the correct code for the illustration,,if not please bear with me, i am still new at posting images here.

Thanks for the very detailed replies. I appreciate it! Lazypup, thanks for the diagram. That was VERY helpful.

I am under the IRC and International Plumbing Code. I can't see the vent because the line for the vent/drain has drywall right up to the opening (no escutcheon). It comes horizontally out of the wall, then turns down and back up.

Everything I wrote presumed your vent was where "we all think it is" but you have to check this and confirm this. It's important. It is quite easy to look deeper into your house and see your vents. You have a roof. Start by looking at your roof. Then, look in your attic. Etc.

The ADA solution I wrote is good. Here is one more term : "offset drain" or "offset handicap sink drain". Search and let us know what you come up with. Using those bends to run the pipe over to where the P trap is now, your P trap doesn't move.

If you move the P trap from where it is now, you still have other constraints to contend with. The height of the P trap has to be within a range, not too high, not too low.

If I were a plumbing code instructor I would have all the right terms and nothing but the right terms, with no extra words added that might confuse. If lazypup corrects a portion of what I write, that is great.

I've seen thousands of plumbing posts from homeowners. Based on your posts I would recommend you not to do anything after this little bit of internet learning. Either get a plumber or post here again many times to ensure that your understanding & implementation correspond to what has been described above by lazyup or by me or by others who will post later.

Given that your are under the IRC the solution to your problem is fairly easy.

in your original post you stated that you want to move your sink about 15" to one side of where the existing sink is. You didn't state whether you were moving to the right or the left, but then that really doesn't matter anyway, except for the sake of illustration allow me to say we are moving to the right. If you are in fact moving to the left the procedure will be the same but in the opposite direction.

We can use the simple triangle formula which states that the length of the hypotenuse of a triangle is equal to the square root of Side A squared plus side B squared.

We are moving 15" horizontal along the wall from the existing fixture arm, so let us say "Side A" =15" and the center of the drain is approximately 12" forward from the wall so let us say "Side B"=12".

If we were certain that there is a vent behind the wall in the manner that i showed in the previous illustration this short length would be of no consequence, however since we don't know whether that vent exists we must then presume that the fixture arm in the wall may turn horizontal and run a short distance to the vent stack. Understanding that the codes are very specific in regards to the maximum length of the fixture arm from the trap weir to the vent we must then assume that an additional 19" could extend us beyond the allowable limit. Fortunately you are under the IRC, which allows us to use an AAV and no one will ever fault your for having additional venting so we may proceed without opening the wall.

You stated that the existing fixture arm exits the wall, then turns down and back up. That in itself is a code violation for two reasons.
1. dropping down then back up creates a trap and the codes prohibit have two successive traps in series.
2. There must be an even 1/4" per foot downward slope from the trap weir to the vent.

I would begin by cutting the existing fixture arm about 1-1/2" from the wall then attach the discharge end of a Wye on the fixture arm with the side opening of the Wye pointing in the direction of your new trap location.
One the end of the Wye I would install a female thread adapter and a cleanout cap.

i would then install a 1/16 street bend in the side opening of the Wye. (NOTE- A 1/16th street bend is a 22.5 degree sanitary elbow that has a female hub on one end and a male spigot on the other end.)
Next cut about a 2" or 3" section of sched.40 PVC and insert one end into the hub on the 1/16th street bend. Position this Wye with the side opening pointing up and install a 18th street bend in the side opening of the second Wye.(The side opening of a Wye comes off at 45deg. adding the 1/8th street bend will then complete the 90deg upward turn. (Note-you must use a wye & 1/8 bend here because the code prohibits a Tee on a horizontal line.)

Cut another 2" length of sched.40 PVC and insert it in the top of the 1/8th bend, then install a female thread adapter on the top end of that PVC nipple. You can now screw an AAV into that thread adapter. NOTE-With the Wye, 1/8th bend, PVC nipple and the thread adapter you should easily have the required 4" from the top of the fixture arm to the base of the AAV.)

Now connect your trap to the sink tailpiece and direct the tailpiece of the trap towards your new fixture arm. You can then compute the final length from the opening of the second Wye to the position where your new Trap adapter will be. (When all is said and done, you may need to cut a bit off the end of the trap tailpiece. that is ok)